This invention relates to a method and apparatus for securing parts, such as webs for wing spars and ribs, to a holding apparatus for machining and/or assembly into a larger part, such as wing spars and ribs, and more particularly to a vacuum head and method for holding a part in a fixed and precisely known position for machining and/or drilling to close tolerances.
Conventional manufacturing techniques for machining and assembling large mechanical assemblies such as airplane wing spars and ribs to a specified contour have, in the past, relied on fixtured xe2x80x9chardpointxe2x80x9d tooling techniques utilizing floor assembly jigs and templates to locate and temporarily fasten detailed structural parts together to position the parts correctly relative to one another. This traditional tooling concept usually requires primary assembly tools for each subassembly produced, and large assembly tools in which the subassemblies are assembled into the assembled structure.
Assembly tooling is intended to accurately reflect the original engineering design of the product, but using the conventional tooling concept in which the tooling sets the configuration of the final assembly, there are many steps between the original design of the product and the final manufacture of the tool. It is not unusual that the tool as finally manufactured produces missized spars or wing components that would be outside of the dimensional tolerances of the original spar or spar component design without extensive, time consuming and costly hand work to correct the tooling-induced errors. More seriously, a tool that was originally built within tolerance can become out of tolerance from the hard use it typically receives in the factory. Moreover, dimensional variations caused by temperature changes in the factory can produce a variation in the final part dimensions as produced on the tool, particularly when a large difference in the coefficient of thermal expansion exists between the tooling and the spar, as in the usual case where the tooling is made of steel and the spar components are made of aluminum. Since dimensions in airplane construction are often controlled to within 0.005xe2x80x3, temperature induced dimensional variations can be significant.
Hand drilling of the part on the tool can produce holes that are not perfectly round or normal to the part surface when the drill is presented to the part at an angle that is slightly nonperpendicular to the part, and also when the drill is plunged into the part with a motion that is not perfectly linear. Parts can shift out of their intended position when they are fastened in non-round holes, and the nonuniform hole-to-fastener interference in a non-round hole lacks the strength and fatigue durability of round holes. The tolerance buildup on the spar subassemblies can result in significant growth from the original design dimensions, particularly when the part is located on the tool at one end of the part, forcing all of the part variation in one direction instead of centering it over the true intended position.
One effective solution to the hard tooling problem is shown in U.S. patent application Ser. No. 09/155,236 entitled xe2x80x9cDeterminant Spar Assemblyxe2x80x9d by Clayton Munk, Paul Nelson and David Strand. The process and apparatus of that invention eliminates hard tool and uses instead a fixture that holds the part in a position that can be probed to determine its actual location in space without reference to the fixture itself. However, it is still desirable to mount the part on the fixture in a position and orientation that is as close to a known and flat shape as possible, and to do so quickly and inexpensively. The use of a vacuum head on the settable pogos of the holding apparatus solves the requirement.
Accordingly, this invention makes it possible to hold parts in precisely known positions for machining and assembly operations. It provides vacuum to a vacuum head only if a part or other tool is presented to the vacuum head to avoid compromising the vacuum source by open vacuum heads, and eliminates the need to manually turn off or seal vacuum heads that are not used to hold a particular part on the holding apparatus.
The benefits of the invention are attained in a method and apparatus for holding a part on a fixture in a precisely known predetermined position. It includes pressing a surface of the part against a vacuum head and sealing an annular elastomeric sealing lip of a vacuum cup against the part surface. A valve button of a valve is engaged by the part surface to open the valve and establish communication between a vacuum source. The part is pulled by the vacuum against a hard stop in the vacuum head having a fixed precisely known relationship to said vacuum head reference surface. The hard-stop can be mounted in the base of the vacuum head for pivoting about at least one axis to self-align the hard-stop face with the part face.